Graphite fiber reinforced epoxy composites are playing an important role in the production of critical aerospace structures and primary structures of commercial aircraft. These graphite/epoxy composite material products are often fabricated from resin-impregnated cloth, mat or filaments in flat form known as prepreg. After the initial fabrication, or layup, process composite materials generally are inspected to identify flaws in the composite material, as well as to verify that the material edges are within tolerance.
For aerospace products made from composite materials it is important that flaws and out of tolerance conditions are prevented from entering the consolidation and cure stages. This is to reduce the cost of repair or complete rejection of the product. Manual inspection to perform this task during layup can be very labor intensive and problematic. Composite material flaws can include wrinkles, disbonds, inclusions, and porosity. Many out of specification conditions such as excessive gaps, splices, resin or fiber cannot be effectively detected after cure by non-destructive techniques and must be prevented or detected for correction during layup.
A method currently used to inspect composite materials involves visual imaging of the material surface. The images are processed to detect shadows and reflections on the surface in order to detect wrinkles, Foreign Object Debris (FOD), and material edges. However, machine clearance and the width of the material can make the desired lighting angle difficult or impossible to obtain. Furthermore, this method is not successful in detecting non-adhered material or inclusions, which produce minimal changes in the composite material surface.
Another method of inspection involves processing a visual image of a laser-produced line to detect variations in the height of the composite material surface in order to detect wrinkles and material edges. However, this method requires precise control of distance to and alignment with the material surface, making the method susceptible to vibrations during fabrication processes. This method also is not successful in detecting non-adhered material or thin inclusions, which produce extremely small height variations.
It would be useful to identify composite material edges on and below the surface and to evaluate material defects and foreign material inclusions with reference to the material edge locations using a process that is not highly sensitive to machine vibration or lighting angle. It would also be useful to perform the inspection and evaluation during the initial material laydown fabrication process in order to reduce rework and prevent product rejection, as well as to provide automatic control of fabrication devices. Accordingly, it is desirable to provide a method and apparatus that perform real-time nondestructive inspection of a composite material during fabrication and provide automatic control of a fabrication device.